In a recent highly information-oriented society, a communication apparatus such as a mobile phone, a wireless LAN, and the like is required to ensure linearity of a transmission signal over a broad power amplification range while operating with low power consumption. In such a communication apparatus, a transmitter which outputs a highly accurate transmission signal regardless of a bandwidth while operating with high efficiency is adopted. In the following, a conventional transmitter will be described.
As a conventional transmitter, for example, there has been a transmitter (hereinafter referred to as a quadrature modulation circuit) which generates a transmission signal by using a modulation method such as a quadrature modulation, and the like. Here, as being widely known, description of the quadrature modulation circuit is omitted. Further, as a conventional transmitter which operates with higher accuracy and higher efficiency than the quadrature modulation circuit, for example, there has been a transmitter 50 shown in FIG. 11.
FIG. 11 is a block diagram showing an example of a configuration of the conventional transmitter 50. In FIG. 10, the conventional transmitter 50 includes a signal generation section 501, a phase modulator 502, a regulator 503, a power amplifier 504, and a power supply terminal 505. The power amplifier 504 includes a transistor for amplification.
In the conventional transmitter 50, the signal generation section 501 generates an amplitude signal and a phase signal. The amplitude signal is inputted to the regulator 503. Further, a direct current voltage is supplied to the regulator 503 from the power supply terminal 505. The regulator 503 supplies, to the power amplifier 504, a voltage in accordance with the inputted amplitude signal. Typically, the regulator 503 supplies the power amplifier 504 with a voltage which is proportional to a magnitude of the inputted amplitude signal.
Meanwhile, the phase signal is inputted to the phase modulator 502. The phase modulator 502 phase-modulates the phase signal to a phase modulated signal and outputs the phase modulated signal. The phase modulated signal is inputted to the power amplifier 504. The power amplifier 504 amplitude-modulates the phase modulated signal to a modulated signal by the voltage supplied from the regulator 503 and outputs the modulated signal which has been phase-modulated and amplitude-modulated. The modulated signal is outputted as a transmission signal from an output terminal. It should be noted that the transmitter 50 described above is referred to as a polar modulation circuit.
In the conventional transmitter 50, the amplitude signal and the phase signal generated by the signal generation section 501 are signal-processed in two paths (a phase modulator 502 and the regulator 503) separately and amplitude-modulated concurrently by the power amplifier 504. Thus, in the power amplifier 504, a difference (that is, a delay error) may occur between a delay time generated in the path of the amplitude signal and a delay time generated in the path of the phase signal. Hereinafter, the delay time generated in the path of the amplitude signal is simply referred to as a delay time of the amplitude signal while the delay time generated in the path of the phase signal is simply referred to as a delay time of the phase signal. The difference between the delay times described above deteriorates distortion of the transmission signal.
Patent Literature 1 discloses a transmitter which compensates a difference between a delay time of a phase signal and a delay time of an amplitude signal in the polar modulation method. FIG. 12 is a block diagram showing a configuration of a conventional transmitter 51 disclosed in Patent Literature 1. In FIG. 12, the conventional transmitter 51 includes an input terminal 511, a signal input section 512, a coupler 513, a limiter 514, a phase shifter 515, a radio-frequency amplifier 516, an envelope detection section 517, an amplitude modulator 518, a coupler 519, an output terminal 520, a phase difference calculation section 521, and a phase shift control section 522.
An input signal from the signal input section 512 is inputted via the coupler 513 to each of the limiter 514 and the envelope detection section 517. The limiter 514 extracts a phase signal from the input signal. The phase signal is inputted via the phase shifter 515 to the radio-frequency amplifier 516. The envelope detection section 517 extracts an amplitude signal from the input signal. The amplitude signal is modulated to a predetermined magnitude by the amplitude modulator 518 and then inputted to the radio-frequency amplifier 516. By amplitude-modulating the phase signal in accordance with the amplitude signal, the radio-frequency amplifier 516 obtains an output signal.
Further, the signal input section 512 generates and outputs test signals Sin1, Sin2 in order to compensate the delay error. To the phase difference calculation section 521, the test signals Sin1, Sin2 are inputted via the coupler 513 while output signals Sout1, Sout2 from the radio-frequency amplifier 516 are inputted via the coupler 519. The phase difference calculation section 521 calculates a phase difference between the test signal Sin1 and the output signal Sout1, and obtains information of the phase difference between the Sin1 and the Sout1. Further, the phase difference calculation section 521 calculates a phase difference between the test signal Sin2 and the output signal Sout2 and obtains information of the phase difference between the Sin2 and the Sout2.
The phase difference calculation section 521 calculates a difference between a delay time of the amplitude signal and a delay time of the phase signal on the basis of the information of the phase difference between the Sin1 and the Sout1 and the information of the phase difference between the Sin2 and the Sout2. By adjusting a phase shift amount of the phase shifter 515 on the basis of the difference, which has been calculated by the phase difference calculation section 521, between the delay time of the amplitude signal and the delay time of the phase signal, the phase shift control section 522 adjusts the difference between the delay time of the amplitude signal and the delay time of the phase signal.
Citation List
[Patent Literature]
[Patent Literature 1] Japanese Laid-Open Patent Publication No. 2008-211449